1. Field of the Invention
This invention relates to power supplies for electronic devices and particularly to circuits to maintain a constant polarity across an electrical load.
2. Description of the Related Art
Electronic devices, and particularly portable electronic devices such as portable computers, cellular phones, and personal digital assistants (PDAs) typically make use of alternating current to direct current adapters ("AC-DC adapters," "AC adapters," or simply "adapters") either as a direct source of power, or as a source of power to charge on-board batteries. AC adapters can be built into such electronic devices, but given the size, weight, and cost constraints often imposed on such devices, AC adapters are more commonly provided as a separate module with a plug or cord for connecting the adapter to an AC outlet, and another cord for connecting the adapter to the electronic device through a connector.
Given the variety of electronic devices that use AC adapters, and the various output polarizations, voltage ratings, and current ratings of those adapters, an electronic device user is likely to have several, if not many, different adapters for different electronic devices. Consequently, matching the correct adapter to the intended device can be difficult because of similarity in appearance among adapters and similarity among the connectors associated with the adapters. Compounding this problem is the fact that adapters intended for different applications can be manufactured by the same company and look the same, yet have dissimilar electrical characteristics. Moreover, using the wrong adapter can damage expensive electronic equipment or even present a safety hazard.
Prior methods to prevent improper use of and/or mitigate the damage from improper use of an AC adapter generally fall into two categories: mechanical methods and electrical methods. The most common mechanical solution to the problem of improper adapter use is to provide the adapter and the electronic device using the adapter with unique connector keying such that the wrong connector cannot be inserted into the electronic device. One drawback to this method of preventing improper use of an adapter is that it prevents manufacturers from using standard connectors and adapters which allow the manufacturer to avoid the high costs of tooling, testing, and providing a custom part.
Electrical solutions typically include circuitry for clamping the improper input voltage with a dissapative device such as a zener diode, a metal oxide varistor (MOV), or a junction diode. These devices will only work if the input power source has power limiting within the capability of the dissapative device, and thus there ability to protect a device is limited. Such specialized circuits or components add cost and complexity to the electronic device. Additionally, operating conditions within the specification of the dissapative device are not always met, so damage to the electronic device can still result from using an improper adapter.
Accordingly, it is desirable to have a circuit that allows power input of various polarities, while supplying power with a constant polarity to an electrical load. Additionally, it is desirable to have such a circuit that can provide power having constant polarity given either AC or DC input power.